32.Characterization, Luminescence and EPR Investigations of Eu2+

Journal of Non-Crystalline Solids 355 (2009) 2491–2495 Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol Characterization, luminescence and EPR investigations of Eu2+ activated strontium aluminate phosphor Vijay Singh a, Jun-Jie Zhu a,*, Manoj Tiwari b, Manish Soni c, Mahendra Aynayas c, Seok-Hee Hyun d, R. Narayanan e, Manoj Mohapatra f, V. Natarajan f a Department of Chemistry, Key Lab of Analytical Chemistry for Life Science, Nanjing University, Nanjing 210093, PR China b Manoharbhai Patel Institute of Engineering and Technology, Kudwa Village, Gondia 441 614, India c Department of Physics, Sadhu Vaswani College, Bairagarh, Bhopal 462 030, India d Centre for Scientific Instruments, Kyungpook National University, Daegu 702701, South Korea e Department of Dental Biomaterials, School of Dentistry, Kyungpook National University, Daegu, South Korea f Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India article info abstract Article history: Strong blue-green light emitting Eu doped SrAl2O4 phosphor was synthesized by a low-temperature ini- Received 1 November 2008 tiated, self-propagating and gas producing combustion process in a very short time (<5 min). The pre- Received in revised form 5 July 2009 pared powder was characterized by X-ray diffraction, Fourier-transform infrared spectrometry and Available online 7 October 2009 scanning electron microscopy. The excitation spectrum shows a peak at 397 nm. Upon excitation at 397 nm, the emission spectrum exhibits a well defined broad band with maximum at 493 nm corre- PACS: sponding to 4f65d ? 4f7 transition. Electron paramagnetic resonance (EPR) measurements at X-band 78.55.-m showed low field signals due to Eu2+ ions in SrAl O :Eu. 76.30.Kg 2 4 Ó 2009 Elsevier B.V. All rights reserved. Keywords: Luminescence Phosphors Electron spin resonance 1. Introduction (SrA12O4) below 1200 °C is quite difficult and there is always possibility of other phases such as SrAl4O7, SrAl12O19,Sr3Al2O6, Rare earth activated inorganic phosphors are widely used in a and Sr4Al14O25 [9,10]. variety of applications, such as lamp industry, color display, radia- The alkaline earth aluminates have been studied for more than tion dosimetry and X-ray imaging. It is also known that the emis- three decades [11,12] for use as luminescent materials or in sion of Eu2+ ions varies from blue to red depending on the host cements. These aluminates have been prepared traditionally by lattice due to crystal-field effects [1]. The luminescent properties solid state reactions [13,14], which, in general, demand high of Eu2+-doped strontium aluminate phosphors have been studied annealing temperatures and long times of firing, ca. 1300– extensively because they show a long anomalous phosphorescence 1600 °C and 5–10 h, respectively. In addition, products can be and/or a short-time decay depending on the conditions of prepara- strongly sintered and the doping luminescent ions may not be tion used [2,3]. Polycrystalline SrAl12O19:Mn is known as a green- homogeneously dispersed. Alternatively low-temperature meth- emitting phosphor for plasma display panels [4] and Pr3+,Nd3+ ods, e.g. sol–gel [15–17], pechini [18], hydrothermal [19] and doped SrAl12O19 crystals show good laser properties [5]. Recently, microwave [20] methods can be used instead of the direct solid luminescence of Eu2+ in some strontium aluminate hosts (e.g., state reaction due to economical interest and to enhance the prop- SrAl2O4, SrAl2B2O7, SrAl4O7, SrAl12O19,Sr3Al2O6, and Sr4Al14O25) erties of materials. However, combustion synthesis studies that co-doped with other rare-earth ions, have attracted much atten- compare the different methods are rare. Therefore, we have syn- tion due to their special long afterglow phenomenon [6–8]. The thesized Eu doped SrAl2O4 material by a low-temperature initiated aluminate of interest in the present study is SrAl2O4. In the past, combustion process. The products were characterized using it has been reported that, to obtain strontium monoaluminate techniques such as powder X-ray diffraction (XRD), scanning electron microcopy (SEM), Fourier-transform infrared (FT-IR) spectra, * Corresponding author. Tel./fax: +86 25 83594976. photoluminescence (PL) and electron paramagnetic resonance E-mail address: [email protected] (J.-J. Zhu). (EPR). 0022-3093/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2009.08.027 2492 V. Singh et al. / Journal of Non-Crystalline Solids 355 (2009) 2491–2495 2. Experimental tude. EPR measurements were carried out using a Bruker EMX 10/ 12 X-band ESR spectrometer. 2.1. Sample preparation 3. Results and discussion Stoichiometric compositions of the metal nitrates (oxidizers) and urea (fuel) were calculated using the total oxidizing (O) and The formation of the crystalline phase of as-prepared products reducing (F) valencies of the components, which serve as the was confirmed by X-ray diffraction. Fig. 1 shows the X-ray patterns numerical coefficients for the stoichiometric balance so that the of SrAl O :Eu powder. The X-ray pattern of combustion synthe- equivalence ratio ue, is unity (i.e. O/F = 1) and the energy released 2 4 sized sample at furnace temperature 500 °C indicated a dominant by the combustion is at a maximum [21]. phase of the standard SrAl O (JCPDS, 74-0794). Beside of SrAl O Analytical grade corresponding metal nitrates (oxidizer), urea 2 4 2 4 peaks there are some other peaks observed which might be corre- (fuel) and europium oxide (activator) were used as the starting sponding to SrO(Al O ) [22,23] and Sr Al O [24] phases. It has materials. The above materials were mixed according to the chem- 2 3 2 3 2 6 been reported in the literature that phase-pure SrAl O could be ical formula Sr Eu Al O , where x = 0.01. For this, we had taken 2 4 1Àx x 2 4 achieved using conventional solid state process, the required tem- 5 g Al(NO ) Á9H O, 1.3962 g Sr(NO ) , 2.6723 g CH N O and 3 3 2 3 2 4 2 perature for synthesizing SrAl O being 1400–1600 °C [25,26]. 0.0117 g Eu O . These were mixed in an agate mortar and the 2 4 2 3 The FT-IR spectrum of SrAl O :Eu powder is shown in Fig. 2. resulting paste was transferred into a china crucible. The crucible 2 4 This spectrum exhibits broad band near 3433 cmÀ1 due to the containing the paste was introduced into a muffle furnace main- OHÀ stretching vibrations of free and hydrogen-bonded hydroxyl tained at 500 °C. Initially, the paste melts and undergoes dehydra- groups. However a weak absorption band at 1632 cmÀ1 appears tion followed by decomposition with the evolution of large from deformative vibration of water molecules, which is probably amounts of gases (oxides of nitrogen). The mixture then froths due to water absorption during the compaction of the powder and swells forming foam, which ruptures with a flame and glows specimens with KBr [27]. The appearance of a very weak band at to incandescence. During incandescence the foam further swells 1382 cmÀ1 is due to the symmetric stretching vibrations of the to the capacity of the container. The entire combustion process is N–O group, which might have resulted from the nitrate of the over in less than 5 min. The dish was then taken out and the foamy product is crushed into fine white powder and was used for characterization without any further heat-treatment. 75 2.2. Instruments 70 Powder samples were analyzed for XRD using a X’Pert PRO- MRD, made in Netherlands. It was used with Cu Ka radiation at 65 40 kV and 40 mA and a scan rate of 0.02°/s in the 2h range from 10° to 70°. The data were collected using the X’Pert Data Collector 60 data acquisition software and analyzed by means of the X’Pert %T HighScore data analysis package. The morphology of the powders 55 was obtained using a Hitachi S-4300 scanning electron microscope (SEM). FT-IR spectra were recorded using a Perkin–Elmer Rx1 50 instrument in the range 4000–400 cmÀ1. Room temperature photoluminescence (PL) of the prepared phosphors was studied using 45 a Hitachi F-4500 fluorescence spectrophotometer. Alternately, the PL and PL decay time measurements were carried out on an Edin- 40 burgh nF900 unit equipped with a Picoquant pulsed diode laser 4000 3500 3000 2500 2000 1500 1000 500 (PDL) as the excitation source. The laser provides a constant exci- Wavenumber (cm-1) tation wavelength of 375 nm. The frequency of the PDL during the decay time measurements was kept at 20 kHz with 5 V ampli- Fig. 2. FT-IR spectrum of SrAl2O4:Eu phosphor at room temperature. Fig. 1. XRD pattern of as-prepared SrAl2O4:Eu phosphor. V. Singh et al. / Journal of Non-Crystalline Solids 355 (2009) 2491–2495 2493 Fig. 3. SEM microphotographs of SrAl2O4:Eu. starting material [28]. The metal–oxygen stretching frequencies in magnified zone b is shown in Fig. 3(C) and (D). It can be observed the range 400–1000 cmÀ1 are associated with the vibrations of Al– from Fig. 3(D), that the pore diameter is about 150 nm. These voids O, Sr–O and Sr–O–Al bonds [29]. The sample showed a strong peak and pores might have been formed by the gases evolved during À1 at 846 cm , assigned to the formation of SrAl2O4 [30]. combustion. The morphology of magnified Fig. 3(E) is shown in Fig. 3 shows the SEM micrographs of SrAl2O4:Eu at different Fig. 3(F). It can be seen from high magnification image that there magnifications.

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